[RFC PATCH v2 07/26] KVM: arm64: Introduce a BSS section for use at Hyp

[Quentin Perret]

Currently, the hyp code cannot make full use of a bss, as the kernel
section is mapped read-only.

While this mapping could simply be changed to read-write, it would
intermingle even more the hyp and kernel state than they currently are.
Instead, introduce a __hyp_bss section, that uses reserved pages, and
create the appropriate RW hyp mappings during KVM init.

Signed-off-by: Quentin Perret <qper...@google.com>
---
 arch/arm64/include/asm/sections.h |  1 +
 arch/arm64/kernel/vmlinux.lds.S   |  7 +++++++
 arch/arm64/kvm/arm.c              | 11 +++++++++++
 arch/arm64/kvm/hyp/nvhe/hyp.lds.S |  1 +
 4 files changed, 20 insertions(+)

diff --git a/arch/arm64/include/asm/sections.h 
b/arch/arm64/include/asm/sections.h
index 8ff579361731..f58cf493de16 100644
--- a/arch/arm64/include/asm/sections.h
+++ b/arch/arm64/include/asm/sections.h
@@ -12,6 +12,7 @@ extern char __hibernate_exit_text_start[], 
__hibernate_exit_text_end[];
 extern char __hyp_idmap_text_start[], __hyp_idmap_text_end[];
 extern char __hyp_text_start[], __hyp_text_end[];
 extern char __hyp_data_ro_after_init_start[], __hyp_data_ro_after_init_end[];
+extern char __hyp_bss_start[], __hyp_bss_end[];
 extern char __idmap_text_start[], __idmap_text_end[];
 extern char __initdata_begin[], __initdata_end[];
 extern char __inittext_begin[], __inittext_end[];
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 43af13968dfd..3eca35d5a7cf 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -8,6 +8,13 @@
 #define RO_EXCEPTION_TABLE_ALIGN       8
 #define RUNTIME_DISCARD_EXIT
 
+#define BSS_FIRST_SECTIONS                             \
+       . = ALIGN(PAGE_SIZE);                           \
+       __hyp_bss_start = .;                            \
+       *(.hyp.bss)                                     \
+       . = ALIGN(PAGE_SIZE);                           \
+       __hyp_bss_end = .;
+
 #include <asm-generic/vmlinux.lds.h>
 #include <asm/cache.h>
 #include <asm/hyp_image.h>
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 3ac0f3425833..51b53ca36dc5 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1770,7 +1770,18 @@ static int init_hyp_mode(void)
                goto out_err;
        }
 
+       /*
+        * .hyp.bss is placed at the beginning of the .bss section, so map that
+        * part RW, and the rest RO as the hyp shouldn't be touching it.
+        */
        err = create_hyp_mappings(kvm_ksym_ref(__bss_start),
+                                 kvm_ksym_ref(__hyp_bss_end), PAGE_HYP);
+       if (err) {
+               kvm_err("Cannot map hyp bss section: %d\n", err);
+               goto out_err;
+       }
+
+       err = create_hyp_mappings(kvm_ksym_ref(__hyp_bss_end),
                                  kvm_ksym_ref(__bss_stop), PAGE_HYP_RO);
        if (err) {
                kvm_err("Cannot map bss section\n");
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp.lds.S 
b/arch/arm64/kvm/hyp/nvhe/hyp.lds.S
index 5d76ff2ba63e..dc281d90063e 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp.lds.S
+++ b/arch/arm64/kvm/hyp/nvhe/hyp.lds.S
@@ -17,4 +17,5 @@ SECTIONS {
                PERCPU_INPUT(L1_CACHE_BYTES)
        }
        HYP_SECTION(.data..ro_after_init)
+       HYP_SECTION(.bss)
 }
-- 
2.30.0.284.gd98b1dd5eaa7-goog